566 EXPLORATION GEOPHYSICS 



Structure 

 At the dam site, the correlation of isolated outcrops with the geophysi- 

 cal studies at the various geophysical stations indicates that the Bearpaw 

 shale is quite uniform and that it dips slightly to the east, in conformity 

 with the recognized regional dip. The geophysical work disclosed no 

 evidence of abnormal structural conditions, such as major folding or 

 faulting on the west abutment. Localized faulting is evidenced by small 

 displacements of seams in the shale (six inches or less). However, as 

 most of the exposures are on steep slopes, many of these minor displace- 

 ments are probably due to hill-side "creep." 



Bedrock Contours and Profiles 

 The bedrock contour map plotted from the geophysical data indicates 

 that the overburden-shale contact is comparatively flat over a large central 

 area, but dips steeply to the east in the western part of the area. There 

 is some evidence that this flat central area may represent a bedrock 

 valley or bench underlying the ridge. This indicated subsurface depres- 

 sion might possibly have existed as a pre-glacial drainage channel. The 

 uniform attitude of the contact underneath this portion of the west abut- 

 ment ridge is probably a unique and local condition. Geophysical results 

 on other parts of the area indicate that the shale surface is quite irregular. 



Ground-Water Table 

 The geophysically-indicated moisture table in the west abutment area 

 is shown in the profile. This moisture table marks the approximate loca- 

 tion of the transition zone between the relatively dry near-surface material 

 and the moisture-impregnated deeper material. 



Comparison of Geophysical Results zuith Logs of Core Holes 

 To provide a basis for correlation of the electrical bedrock determina- 

 tions, preHminary studies were conducted at existing core hole locations 

 for which logs were available. As a consequence of these preliminary 

 studies, direct correlation could be made between the electrically indi- 

 cated depth to firm shale and the depth as indicated by drillers' logs. 

 The electrical studies were then utilized to extend the depth determinations 

 into adjacent areas. 



The close check between the electrical determinations and drill hole 

 results is shown on Profile A- A' on which subsequently determined bed- 

 rock depths are plotted for drill holes 58, 387, and 383. 



Gillespie Dam 



The survey* conducted on the Gila River is an example of the use of the 

 electrical method to investigate complex local geology at the site of the 



* For permission to publish this material, the author is indebted to Raymond A. 

 Hill, Supervising Engineer of Quinton, Code and Hill ; Leeds and Barnard, Engineers, 

 Consolidated, Los Angeles; Salt River Valley Water Users Association, Phoenix, 

 Arizona. 



